This invention relates to an apparatus for measuring capacitance, especially large capacitance. The resulting measurement is displayed on a meter calibrated in capacitance.
One method of determining the capacitance of a large, unknown capacitor is to charge it to a predetermined DC voltage, then discharge it through a known resistor and an SCR while observing the discharge wave form on an oscilloscope, then calculating the capacitance from the time for it to discharge to 37% of its charge voltage. This method does take a set amount of time and some accuracy may be lost in determining the time required for the wave form to reach 37% of the charge voltage.
Another method is to insert the unknown capacitor in a bridge circuit in which fixed capacitors and, in some cases, other circuit elements are provided. The measurement is obtained either by reading a meter indicating the imbalance in the bridge, or by adjusting one of the capacitors in the bridge until a balanced condition is reached and then reading the desired measurement from a calibrated knob. This method can give quite precise results; however, a fine touch is needed to tune the knob, and the resulting accuracy depends on the care taken by the operator.